Because JavaScript _is_ the language and people know it. I never understood the concept of a "React developer", for example, although I saw many junior devs who were very well-versed in React and didn't completely understand JavaScript.

In the end, it's a design choice. Of course frameworks don't inherently _need_ to be beholden to the standards of the underlying language, but I think this is just simpler, therefore a worthy goal to pursue.